Polymers, including homopolymers and copolymers, which are both biocompatible and absorbable in vivo are known in the art. Such polymers are typically used to manufacture medical devices which are implanted in body tissue and absorb over time. Examples of medical devices manufactured from these absorbable biocompatible polymers include suture anchor devices, sutures, staples, surgical tacks, clips, plates and screws, etc.
Absorbable, biocompatible polymers useful for manufacturing medical devices include both natural and synthetic polymers. Natural polymers include cat gut, cellulose derivatives, collagen, etc. Synthetic polymers may consist of various aliphatic polyesters, polyanhydrides, poly(orthoester)s, and the like. Natural polymers typically absorb by an enzymatic degradation process in the body, while synthetic absorbable polymers generally degrade primarily by a hydrolytic mechanism.
Synthetic absorbable polymers which are typically used to manufacture medical devices include homopolymers such as poly(glycolide), poly(lactide), poly(e-caprolactone), and poly(p-dioxanone) and copolymers such as poly(lactide-co-glycolide), poly(e-caprolactone-co-glycolide), and poly(glycolide-co-trimethylene carbonate). The polymers may be statistically random copolymers, segmented copolymers, block copolymers, or graft copolymers. It is also known that both homopolymers and copolymers can be used to prepare blends.
U.S. Pat. Nos. 4,653,497, 4,838,267, 5,080,665 describe several biocompatible, absorbable, poly(p-dioxanone-co-glycolide) copolymers useful as biomedical devices. U.S. Pat. Nos. 4,838,267 and 5,080,665 additionally describe poly(p-dioxanone-co-glycolide) block or graft copolymers.
Furthermore, U.S. Pat. No. 4,838,267 describes (See FIGS. 1 and 2) the preparation of poly(p-dioxanone-b-glycolide) block copolymers by a two-step, two-reaction vessel process in which preformed, high molecular weight poly(p-dioxanone), that is substantially free of p-dioxanone monomer, is reacted with glycolide monomer at temperatures from about 140.degree. C. to about 240.degree. C. to yield block copolymers of the (A--B).sub.n type where A is a long block of repeating units of p-dioxanone (i.e., the homopolymer of poly(p-dioxanone)) and B is a long block of repeating units of glycolide (i.e., the homopolymer of poly(glycolide)). The repeating unit structure as well as a schematic representation of the block copolymer structure are shown in FIGS. 1 and 2. These copolymers are highly crystalline, due to their blocky structure, yielding materials with long breaking strength retention profiles (BSR), high strength and relatively high stiffness. It should be noted that breaking strength retention is a conventionally known standard method of measuring the strength of a device made of a bioabsorbable polymer, as a function of time under biological conditions in vitro or as a function of time after being implanted in vivo.
Additionally, U.S. Pat. No. 5,080,665 describes block or graft copolymers of poly(p-dioxanone-co-glycolide) prepared by a process in which the p-dioxanone monomer is reacted initially for a certain period of time, typically one hour at about 180.degree. C., followed by reaction with glycolide at about 200.degree. C. This process leads to block or graft copolymers which are useful due to their formation of a "hard" phase formed from the glycolide repeating unit blocks, and a "soft" phase formed from the p-dioxanone repeating unit blocks as illustrated in FIGS. 3 and 4.
Furthermore, U.S. Pat. No. 4,653,497 describes poly(p-dioxanone)-rich segmented copolymers comprising about 70 weight percent to about 97 weight percent polymerized p-dioxanone with the remaining small portion of the copolymer polymerized with glycolide as illustrated in FIGS. 5 and 6.
Although the above described copolymers yield materials with excellent properties such as high strength and stiffness and long BSR profiles as found with the block copolymers, or good strength, long elongations, low stiffness and shorter BSR profiles as found for the poly(p-dioxanone)-rich segmented copolymers, there is a need in this art for new copolymer compositions having characteristics of both the block copolymers and the segmented copolymers.
Accordingly, what is needed in this art are novel copolymer compositions which have the block copolymer characteristics of high strength and stiffness, and the segmented copolymer characteristics of shorter BSR profiles and absorptions rates.
In certain biomedical applications there is a strong need for these requirements, including sutures with strong tensile properties, like those of Vicryl.RTM. or Dexon.RTM., but shorter BSR profiles and absorption rates.
In addition, it would be highly desirable to have such polymers having little or no unreacted monomers present, since it is believed that the presence of certain levels of unreacted monomers can lead to problems such as adverse tissue reaction.